New evidence for upper Permian crustal growth below Eifel, Germany, from mafic granulite xenoliths

Granulite xenoliths from the Quaternary West Eifel Volcanic Field in Germany record evidence of magmatism in the lower crust at the end of the Permian. The xenoliths sampled two distinct bodies: an older intrusion (ca. 264 Myr old) that contains clinopyroxene with flat, chondrite-normalised rare earth element (REE) profiles and a younger (ca. 253 Myr old) intrusion that crystallised middle-REE-rich clinopyroxene. The younger body is also distinguished based on the negative Sr, Zr and Ti anomalies in primitive mantle-normalised multi-element plots. REE-in-plagioclase–clinopyroxene thermometry records the magmatic temperature of the xenoliths (1100–1300 ∘C), whereas Mg-in-plagioclase and Zr-in-titanite thermometry preserve an equilibration temperature of ca. 800 ∘C. These temperatures, together with a model of the mineral assemblages predicted from the composition of one of the xenoliths, define the pressure of crystallisation as ∼1 GPa. The xenoliths also preserve a long history of reheating events whose age ranges from 220 to 6 Myr. The last of these events presumably led to breakdown of garnet; formation of symplectites of orthopyroxene, plagioclase and hercynite; and redistribution of heavy rare earth elements into clinopyroxene. The data from the West Eifel granulite xenoliths, when combined with the existing data from granulites sampled in the East Eifel, indicate that the lower crust has a long a complex history stretching from at least 1.6 Ga with intrusive events at ca. 410 and 260 Ma and reheating from the Triassic to late Miocene.

Combining volatiles measurements in fluid inclusions with petrology of ultramafic xenoliths: new insights on the evolution of the West Eifel and Siebengebirge (Germany) Sub-Continental Lithospheric Mantle

<p>Integrating petrography and mineral chemistry data with the determination of volatiles concentration and isotopic fingerprint in fluid inclusions (FI) in ultramafic xenoliths opens a new window on the study of the Sub-Continental Lithospheric Mantle (SCLM). This frontier approach is crucial for understanding nature, evolution and volatiles recycling within the lithosphere, being particularly important in active or dormant volcanic areas, where the signature of the surface gaseous emissions can be compared to that of the deep mantle domains.</p><p>Five distinct populations of ultramafic xenoliths brought to the surface in West Eifel (~0.5-0.01 Ma) and Siebengebirge (~30-6 Ma) volcanic fields (Germany) were investigated by combining petrographic and mineral chemistry analyses with noble gases + CO<sub>2</sub> determinations in olivine-, orthopyroxene- and clinopyroxene-hosted FI. Xenoliths from West Eifel are modally and compositionally heterogeneous, as testified by the large forsterite range of olivine, the Cr# range of spinel and the variable Al and Ti contents of pyroxene. Siebengebirge rocks, on the other hand, are quite homogeneous, having mostly refractory composition and reflecting high extents (up to 30%) of melt extraction. Equilibration temperatures vary from 900 to 1180 °C in West Eifel and from 880 to 1060°C in Siebengebirge xenoliths, at comparable oxygen fugacity values. In all xenoliths populations, FI composition is dominated by CO<sub>2</sub>, with olivines being the most gas-poor phases and reflecting a residual mantle that experienced one or more melt extraction episodes. The <sup>3</sup>He/<sup>4</sup>He ratio corrected for air contamination (Rc/Ra values) in all phases varies from 6.8 Ra in harzburgites to 5.5 Ra in lherzolites and cumulates rocks, suggesting a progressive modification of an original MORB-like mantle signature via interaction with crustal-related components with <sup>3</sup>He/<sup>4</sup>He and <sup>4</sup>He/<sup>40</sup>Ar* signature similar to magmatic gaseous emissions. The mineral phase major element distribution, together with the systematic variations in FI composition, the positive correlation between Al-enrichment in pyroxene and equilibration temperatures, and the concomitant Rc/Ra decrease at increasing temperature, suggest that the SCLM beneath Siebengebirge represented the German lithosphere prior to the massive infiltration of melts/fluids belonging to the Quaternary Eifel volcanism. On the other hand, West Eifel xenoliths bear witness of multiple heterogeneous metasomatism/refertilization events that took place in the German SCLM between ~6 and ~0.5 Ma. According to Ne and Ar isotope systematics, the FI composition in the studied xenoliths can be explained by mixing between recycled air and a MORB-like mantle, being irreconcilable with the presence of a lower mantle plume beneath the Central European Volcanic Province.</p>

Geochemistry of noble gas and CO2 in fluid inclusions from lithospheric mantle beneath Eifel and Siebengebirge (Germany)

<p>The study of fluid inclusions (FI) composition (He, Ne, Ar, CO<sub>2</sub>) integrated with the petrography and mineral chemistry of mantle xenoliths representative of the Sub Continental Lithospheric Mantle (SCLM) is a unique opportunity for constraining its geochemical features and evaluating the processes and the evolution that modified its original composition. An additional benefit of this type of studies is the possibility of better constraining the composition of fluids rising through the crust and used for volcanic or seismic monitoring.  </p><p>In this respect, the volcanic areas of Eifel and Siebengebirge in Germany represent a great opportunity to test this scientific approach for three main reasons. First, these volcanic centers developed in the core of the Central European Volcanic Province where it is debated whether the continental rift was triggered by a plume (Ritter, 2007 and references therein). Second, Eifel and Siebengebirge formed in Quaternary (0.5-0.01 Ma) and Tertiary (30-6 Ma), respectively, thus spanning a wide range of age. Third, Eifel is characterized by the presence of CO<sub>2</sub>-dominated gas emissions and weak earthquakes that testify that local magmatic activity is nowadays dormant, but not ended (e.g., Bräuer et al., 2013). It is thus important to better constrain the noble gas signature expected in surface gases in case of magmatic unrest.</p><p>This work focuses on the petrological and geochemical study of mantle xenoliths sampled in the West Eifel and Siebengebirge volcanic areas (Germany) and aims at enlarging the knowledge of the local SCLM. Gautheron et al. (2005) carried out the first characterization of noble gases in FI of crystals analyzed by crushing technique (as in our study) but limited to olivines and to West Eifel eruptive centers. Here, we integrate that study by analyzing olivines, orthopyroxenes and clinopyroxenes from a new suite of samples and by including two eruptive centers from Siebengebirge volcanic field (Siebengebirge and Eulenberg quarries).</p><p>Xenoliths from the Siebengebirge localities are characterized by the highest Mg# for olivine, clinopyroxene and Cr# for spinel, together with the lowest Al<sub>2</sub>O<sub>3</sub> contents for both pyroxenes, suggesting  that the mantle beneath Siebengebirge experienced high degree of melt extraction (up to 30%) while metasomatic/refertilization events were more efficient in the mantle beneath West Eifel.</p><p>In terms of CO<sub>2</sub> and noble gas concentration, clinopyroxene and most of the orthopyroxene show the highest gas content, while olivine are gas-poor. The <sup>3</sup>He/<sup>4</sup>He varies between 5.5 and 6.9 Ra. These values are comparable to previous measurements in West Eifel, mostly within the range proposed for European SCLM (6.3±0.4 Ra), and slightly below that of MORB (Mid-Ocean Ridge Basalts; 8±1Ra). The Ne and Ar isotope ratios fall along a binary mixing trend between air and MORB-like mantle. He/Ar* in FI and Mg# and Al<sub>2</sub>O<sub>3</sub> content in minerals confirm that the mantle beneath Siebengebirge experienced the highest degree of melting, while the metasomatic/refertilization events largely affected the Eifel area.</p><p>References</p><p>Bräuer, K., et al. 2013. Chem. Geol. 356, 193–208.</p><p>Gautheron, C., et al. 2005. Chem. Geol. 217, 97–112.</p><p>Ritter, J.R.R., 2007. In: Ritter, J.R.R., Christensen, U.R. (Eds.), Mantle Plumes: A Multidisciplinary Approach. Springer-Verlag, Berlin Heidelberg, pp. 379–404.</p>

Timescales from mixing to eruption in alkaline volcanism in the Eifel volcanic fields obtained from sanidine and olivine diffusion modelling

<p>Diffusion profiles in sanidine (Ba) and olivine (Mg-Fe, Ca, Mn, and Ni) were used to track recharge events prior to the eruption of the Laacher See volcano, East Eifel volcanic field, western Germany (12.9 ka). Sanidine crystals were analyzed in samples from cumulates and mafic to intermediate phonolites. Olivine crystals occur only in the final mafic eruption products of the compositionally zoned tephra deposit and represent the hybrids of mixing between differentiated phonolite, crystal cumulates, and intruding basanitic magma at the bottom of the magma reservoir. This mixing event is likely related to the eruption triggering event. Additionally, olivine crystals from ten basanitic scoria and maar deposits in the East Eifel and two locations in the West Eifel (Pulvermaar melilith-nephelinite, Meerfelder Maar ol-nephelinite) were analyzed to represent Quaternary parent mafic magmas in Eifel volcanism.</p><p>Olivine from the mafic component that mixed with the Laacher See phonolite are always reversely zoned from cores of variable composition (Fo<sub>83-89</sub>). Zoning of all crystals show trends to a common rim composition (Fo<sub>87.5-89</sub>). Most crystals show additionally a narrow (<10 μm) normally zoned overgrowth at the outermost grain boundary (Fo<sub>86.5-87.5</sub>). Olivine crystals from mafic cones in the East Eifel show similar zoning patterns and core compositions (Fo<sub>80-88</sub>) as those from Laacher See hybrids, but their rims are more variable and always less forsteritic (Fo<sub>83-88</sub>). The lack of olivine rims with >Fo<sub>88</sub> indicates that East Eifel basanites are less primitive than the basanite that intruded into the Laacher See reservoir with olivine rim composition >Fo<sub>89</sub>. However, olivine in samples from the West Eifel nephelinite maar deposits show rim compositions similar to the olivines from Laacher See (Fo<sub>87.5-90</sub>), but are dominantly normal zoned and have high-Fo cores (Fo<sub>88-92</sub>).</p><p>We interpret these observations to indicate that olivine crystals on Laacher See hybrids probably originate from a cumulate or crystal mush with low melt fraction that was disaggregated by the ascending basanite before hybridization. Diffusion modeling of olivine rims indicate a time scale between mixing and eruption of less than 49 days.</p><p>Diffusion times of the sanidine phenocrysts from the intermediate phonolite indicate older recharge events every 1500-3000 yrs that did not result in complete hybridization and eruption. Ba-diffusion times are much shorter for sanidines from the mafic phonolite (4-8 yrs) and the cumulates (months). The reactivation of crystals from cumulates, that can be related to the eruption-triggering recharge event, occurred therefore only months prior to the eruption of Laacher See. These timescales between recharge and eruption are remarkably shorter than the diffusion times calculated for olivine from basanite erupted from scoria cones (up to 500 days).</p>